As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2010 and are well known to the person skilled in the art.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated. The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to about 0.05% Cr may include an alloy having no Cr.
Heat exchangers and other similar equipment, such as condensers, evaporators and the like for use in car coolers, air conditioning systems, industrial cooling systems, etc. usually comprise a number of heat exchange tubes arranged in parallel between two headers, each tube joined at either end to one of the headers. Corrugated fins are disposed in an airflow clearance between adjacent heat exchange tubes and are brazed to the respective tubes.
The fin material or fin stock for brazed heat exchangers is typically fabricated from 3xxx series aluminium alloys such as, for example AA3003 or AA3003 with a purposive addition of Zn up to about 3%.
The fin stock material is joined to the heat exchange tubes in a brazing operation employing an aluminium alloy brazing filler, most commonly made from an AA4xxx-series alloy. The brazing filler is typically present on the outersurface of the heat exchange tube, but also the use of a clad fin configuration alone is being employed.
The most used brazing processes used on an industrial scale are vacuum brazing and controlled atmosphere brazing using a salt-based brazing flux material.
Vacuum brazing is carried out at relatively low atmosphere pressure in the order of about 1×10−5 mbar or less, and is an essentially discontinuous process and puts high demands on material cleanliness. To obtain the optimum conditions for joining to take place, aluminium alloys commonly used for vacuum brazing contain purposive additions of Mg of 1% or more. The Mg destroys the hard oxide film of the filler alloy when it evaporates from the brazing sheet during brazing, and further the evaporated Mg plays the role as getter that removes oxygen and moisture remaining in the brazing furnace. There is always more magnesium present in the furnace then necessary. The excess magnesium condenses on the cold sports in the vacuum furnace and has to be removed frequently. The capital investment for suitable equipment is relatively high.
NOCOLOK™ (registered trademark of Alcan) flux brazing has been used as the principal brazing process to braze automotive heat exchangers by many heat exchanger manufacturers. Major problems that have arisen from the NOCOLOK process have been flux costs, flux handling and the damage flux causes to the furnaces. Also, in complex shaped assemblies the application of the non-corrosive brazing flux prior to brazing at the interior of the assemblies is often considered very difficult and problematic. Consequently, most of the heat exchanger manufacturers have been trying to reduce flux consumption.
There is room for improvement in the art of aluminium alloy fin stock for brazed heat exchangers and for methods of manufacturing such brazed heat exchangers, in particular for brazing methods that do not require the application of a brazing flux material.